What is the name of a cipher that encrypts text by applying a series of alphabetical shifts based on a keyword?

What is the name of a cipher that encrypts text by applying a series of alphabetical shifts based on a keyword?

• A. Vigenère cipher
• B. Caesar cipher
• C. RSA
• D. One-time pad

Answer: (A)

Using a keyword to drive a sequence of alphabet shifts is the Vigenère cipher. In this method, each letter of the plaintext is shifted forward by an amount determined by the corresponding letter in the keyword (A=0, B=1, ..., Z=25). The keyword is repeated to cover the entire message, so the amount of shift changes with each position, creating multiple alphabets used across the text. This polyalphabetic substitution makes frequency analysis harder than a single, fixed shift. This approach is different from a Caesar cipher, which applies one fixed shift to every letter. It’s not RSA, which is a public-key cryptosystem based on number theory, nor a one-time pad, which uses a truly random key of the same length as the message and, if used correctly, is theoretically unbreakable. The keyword-driven series of shifts is the hallmark of the Vigenère cipher.

Using a keyword to drive a sequence of alphabet shifts is the Vigenère cipher. In this method, each letter of the plaintext is shifted forward by an amount determined by the corresponding letter in the keyword (A=0, B=1, ..., Z=25). The keyword is repeated to cover the entire message, so the amount of shift changes with each position, creating multiple alphabets used across the text. This polyalphabetic substitution makes frequency analysis harder than a single, fixed shift.

This approach is different from a Caesar cipher, which applies one fixed shift to every letter. It’s not RSA, which is a public-key cryptosystem based on number theory, nor a one-time pad, which uses a truly random key of the same length as the message and, if used correctly, is theoretically unbreakable. The keyword-driven series of shifts is the hallmark of the Vigenère cipher.